Method for forming a thin-film transistor

Abstract

A method for forming a thin film transistor (TFT) is disclosed. A gate electrode, insulating layer, semiconductor layer, doped silicon layer and metal layer are formed on a substrate. A first photoresist layer with a first absorptivity is formed on the metal layer. A second photoresist layer with a second absorptivity is formed on the first photoresist layer. The second absorptivity is higher than the first absorptivity. An exposure process and a development process are performed to form a first pattern on the first photoresist layer and a second pattern on the second photoresist layer at the same time. An etching process is then performed to transfer the first pattern into the semiconductor layer, the doped silicon layer and the metal layer and transfer the second pattern into the doped silicon layer and the metal layer. After performing the etching process, the first photoresist layer and the second photoresist layer are removed.

What is claimed is:  
     
       1. A method for forming a thin film transistor (TFT), comprising the steps of: 
       (a) forming a gate electrode on a substrate;  
       (b) forming an insulating layer on the gate electrode;  
       (c) forming a semiconductor layer on the insulating layer;  
       (d) forming a doped silicon layer on the semiconductor layer;  
       (e) forming a metal layer on the doped silicon layer;  
       (f) forming a first photoresist layer on the metal layer;  
       (g) forming a second photoresist layer on the first photoresist layer;  
       (h) performing an exposure process and a development process to form a first pattern on the first photoresist layer and a second pattern on the second photoresist layer at the same time;  
       (i) performing an etching process to transfer the first pattern into the semiconductor layer, the doped silicon layer and the metal layer, and transfer the second pattern into the doped silicon layer and the metal layer; and  
       (j) removing the first photoresist layer and the second photoresist layer.  
     
     
       2. The method as claimed in  claim 1 , wherein the first photoresist layer has a first absorptivity, the second photoresist layer has a second absorptivity, and the first absorptivity is lower than the second absorptivity. 
     
     
       3. The method as claimed in  claim 2 , wherein the first absorptivity is 0.2˜0.8 times the second absorptivity. 
     
     
       4. The method as claimed in  claim 1 , wherein the first photoresist layer has a first photosensitivity, the second photoresist layer has a second photosensitivity, and the first photosensitivity is lower than the second photosensitivity. 
     
     
       5. The method as claimed in  claim 1 , further comprising a step (f′) for forming an adhesion layer between the first photoresist layer and the second photoresist layer. 
     
     
       6. The method as claimed in  claim 5 , wherein the adhesion layer is made of hexamethyldisilazane (HMDS). 
     
     
       7. The method as claimed in  claim 1 , wherein the exposure process in the step (h) is selected from a group of a multiple exposure method, a halftone mask exposure method, and a slit mask exposure method. 
     
     
       8. The method as claimed in  claim 1 , wherein the etching process of the step (i) comprises the steps of: 
       (i1) etching the semiconductor layer, the doped silicon layer, and the metal layer by using the first photoresist layer and the second photoresist layer as a mask, so as to transfer the first pattern on the semiconductor layer, the doped silicon layer, and the metal layer;  
       (i2) using the second photoresist layer as a mask to transfer the second pattern on the first photoresist layer; and  
       (i3) etching the doped silicon layer and the metal layer by using the first photoresist layer with the second pattern as a mask, so as to transfer the second pattern on the doped silicon layer and the metal layer.  
     
     
       9. A method for forming an element, comprising the steps of: 
       (a) forming a first photoresist layer on a layer to be etched;  
       (b) forming a second photoresist layer on the first photoresist layer;  
       (c) performing an exposure process and a development process to form a first pattern on the first photoresist layer and a second pattern on the second photoresist layer at the same time;  
       (d) performing an etching process to transfer the first pattern and the second pattern into the layer; and  
       (e) removing the first photoresist layer and the second photoresist layer.  
     
     
       10. The method as claimed in  claim 9 , wherein the first photoresist has a first absorptivity, the second photoresist has a second absorptivity, and the first absorptivity is lower than the second absorptivity. 
     
     
       11. The method as claimed in  claim 10 , wherein the first absorptivity 0.2˜0.8 times the second absorptivity. 
     
     
       12. The method as claimed in  claim 9 , wherein the first photoresist layer has a first photosensitivity, the second photoresist has a second photosensitivity, and the first photosensitivity is lower than the second photosensitivity. 
     
     
       13. The method as claimed in  claim 9 , further comprising a step (a′) for forming an adhesion layer between the first photoresist layer and the second photoresist layer. 
     
     
       14. The method as claimed in  claim 13 , wherein the adhesion layer is made of hexamethyldisilazane (HMDS). 
     
     
       15. The method as claimed in  claim 9 , wherein the exposure process of the step (c) is selected from a group of a multiple exposure method, a halftone mask method, and a slit mask exposure method. 
     
     
       16. The method as claimed in  claim 9 , wherein the etching process of the step (d) comprises the steps of: 
       (d1) etching the layer by using the first photoresist layer and the second photoresist layer as a mask, so as to transfer the first pattern on the layer;  
       (d2) using the second photoresist layer as a mask to transfer the second pattern on the first photoresist layer; and  
       (d3) etching the layer by using the first photoresist layer with the second pattern as a mask, so as to transfer the second pattern on the layer.